<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    #div1{
      width:400px;
      height:400px;
      background:white;
      position: fixed;
      top: 150px;
      left: 150px;
    }
    body{
      margin:0;
      background: black;
    }
  </style>
</head>
<body>
<div id="div1">
  <!--
  cx、cy是基于父元素的
  -->
  <svg xmlns="http://www.w3.org/2000/svg" width="100%" height="100%">
    <circle cx="100" cy="100" r="40" fill="red"></circle>
  </svg>
</div>
</body>
</html>
